The low molecular weight polyethylene market is spanned by products having molecular weights varying from just a few hundred to several thousand. Polymers having weight average molecular weights of from about 2,000 to about 4,000 are generally classified as waxes, while polymers having weight average molecular weights of from about 4,000 to about 14,000 are generally classified as wax-resins. The waxes have melt indices varying from about 1000 g/10 minutes to about 2500 g/10 minutes, while the wax-resins have melt indices varying from about 500 g/10 minutes to about 1000 g/10 minutes.
Polyethylene waxes and wax-resins are conventionally prepared by the high pressure (&gt;100,000 kPa) homopolymerization of ethylene in stirred and elongated tubular reactors in the absence of solvent using free radical initiators. However, the elevated pressure required to produce these low molecular weight materials by this technique necessitates a high level of energy consumption which materially affects the manufacturing cost of these products.
Gas-phase, fluidized bed processes, such as described in U.S. Pat. Nos. 4,302,565 and 4,302,566, are well known cost effective methods of producing high modulus, high molecular weight ethylene copolymers having a density of from 0.91 g/cm.sup.3 to 0.96 g/cm.sup.3. However, such processes have not heretofore been employed to produce very low molecular weight products such as waxes and wax-resins. The reason for this is that low molecular weight materials of this type have low sintering temperatures which causes the polymer particles to soften and stick together at the reactor temperatures normally employed in fluidized bed polymerizations. As a result of this particle agglomeration, fluidization soon ceases and polymerization comes to a halt due to reactor fouling.
European patent 0 120 053 discloses that low modulus ethylene copolymers having a density of less than 0.91 g/cm.sup.3 can be prepared in gas phase in a fluidized bed using a titanium-based catalyst provided that a large volume of a diluent gas is present in the reaction mixture. According to this reference, hydrogen may be employed as a diluent gas in a mol ratio of hydrogen to ethylene of from 0.01:1 to 0.5:1. In this process, the hydrogen acts not only as a diluent, but also as a chain transfer agent to regulate the molecular weight of the copolymers produced by the process. Copolymers having a melt index of from greater than 0 g/10 minutes to about 25 g/10 minutes are produced by the process.
European patent 0 120 501 discloses that ethylene copolymers having a density of less than 1.91 g/cm.sup.3 can also be prepared in gas phase in a fluidized bed using a vanadium-based catalyst by following the procedure of European patent 0 120 053, i.e., by employing a large volume of a diluent gas in the reaction mixture. According to this reference, hydrogen may be employed in a mol ratio of hydrogen to ethylene of from 0.001:1 up to 2.0:1. However, this reference only discloses the preparation of copolymers having a melt index up to 300 dg/min., preferably from 0.1 dg/min. to 50 dg/min, and, like European patent 0 120 053, does not contemplate the production of very low molecular weight copolymers.
Attempts have been made to produce very low molecular weight waxes and wax-resins in a fluidized bed by modifying the procedure of European patent 0 120 053, e.g., by increasing the concentration of hydrogen in the reactor. Hydrogen, of course, is a well known chain transfer agent and is frequently employed in ethylene polymerizations to control molecular weight. However, it has been found that catalyst activity is adversely affected at the high concentrations of hydrogen required to produce the desired products (at least 60 mol percent hydrogen in the reaction mixture), and if the concentration of hydrogen exceeds 50 mol percent, catalyst activity virtually ceases and polymerization in effect comes to a halt.
Copending application Ser. No. 458,343 of M. C. Hwu et al., filed Dec. 28, 1989, discloses that ethylene can be successfully polymerized with at least one higher alpha-olefin by means of a titanium-based catalyst in a fluidized bed in the presence of high concentrations of hydrogen to produce ethylene copolymers having a melt index of from about 500 g/10 minutes to about 2500 g/10 minutes, while still maintaining satisfactory catalyst activity, provided that polymerization is first effected in the presence of a hydrogen concentration of less than 50 mol percent before the hydrogen concentration is increased to the level required to produce the desired products.